Atrial fibrillation (often referred to simply as AF), in its chronic and paroxysmal forms, constitutes the arrhythmia that is most frequent in the population, with a particularly high incidence (10%) in adults aged over 65 years. There is no pharmacological treatment which is sure to be effective and, as for all antiarrhythmic drugs, there is an increased probability of inducing serious ventricular proarrhythmia. In patients who are suffering from this condition to an incapacitating extent and who do not respond to the use of an average of two to three antiarrhythmia drugs, the subject of an alternative solution becomes important.
An implantable atrial defibrillator with an associated ventricular stimulation/sensing capability constitutes a recent treatment which is currently at the clinical evaluation stage. A defibrillator of this type is usually implanted by the insertion in the heart of two electrodes (leads) having large active surface areas and constituted by metal coils which are positioned along the outer wall of the right atrium and in the coronary sinus, whilst the metal container of the defibrillator can also operate as an active pole during discharge.
The basic object of defibrillation is to produce an electric field adequate to involve, anatomically, a significant portion of the myocardium of both atria, in order to depolarize, by means of the electric shock, a predominant number of myocardial cells which are subject to spontaneous, chaotic and non-synchronized electrical activity. Recent tests have shown the great importance of the distribution of the electric field in minimizing defibrillation energy and consequently voltage, muscle-stimulation and perception of pain, which constitutes the most important factor limiting the use of these devices.